fix(security): 전체 서비스 보안 점검 — XSS·인가·토큰·헤더·에러마스킹 일괄 수정

Phase 1 CRITICAL XSS:
- marked.parse() → DOMPurify.sanitize() (system3 ai-assistant, issues-management)
- toast innerHTML에 escapeHtml 적용 (system1 api-base, system3 common-header)
- onclick 핸들러 → data 속성 + addEventListener (system2 issue-detail)

Phase 2 HIGH 인가:
- getUserBalance 본인확인 추가 (tksupport vacationController)

Phase 3 HIGH 토큰+CSP:
- localStorage 토큰 저장 제거 — 쿠키 전용 (7개 서비스)
- unsafe-eval CSP 제거 (system1 security.js)

Phase 4 MEDIUM:
- nginx 보안 헤더 추가 (8개 서비스)
- 500 에러 메시지 마스킹 (5개 API)
- path traversal 방지 (system3 file_service.py)
- cookie fallback 데드코드 제거 (4개 auth.js)
- /login/form rate limiting 추가 (sso-auth)

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

system1-factory/web/js/api-base.js

@@ -193,7 +193,7 @@ if ('caches' in window) {
     var iconMap = { success: '\u2705', error: '\u274C', warning: '\u26A0\uFE0F', info: '\u2139\uFE0F' };
     var toast = document.createElement('div');
     toast.className = 'toast toast-' + type;
-    toast.innerHTML = '<span class="toast-icon">' + (iconMap[type] || '\u2139\uFE0F') + '</span><span class="toast-message">' + message + '</span>';
+    toast.innerHTML = '<span class="toast-icon">' + (iconMap[type] || '\u2139\uFE0F') + '</span><span class="toast-message">' + escapeHtml(message) + '</span>';
     container.appendChild(toast);
 
     setTimeout(function() { toast.classList.add('show'); }, 10);